CN111052605A

CN111052605A - Furnace frequency control reference oscillator and manufacturing method thereof

Info

Publication number: CN111052605A
Application number: CN201880056339.XA
Authority: CN
Inventors: A·奥加; A·贾克拉
Original assignee: Jingci Diketing Co
Current assignee: Jingci Diketing Co
Priority date: 2017-09-05
Filing date: 2018-09-05
Publication date: 2020-04-21
Anticipated expiration: 2038-09-05
Also published as: US11190133B2; EP3679652A4; US20210135626A1; EP3679652A1; CN111052605B; JP2020532930A; KR20200049802A; WO2019048737A1; FI20175792A1; FI128032B; JP7256806B2

Abstract

The invention relates to a temperature compensation micro-electromechanical oscillator and a manufacturing method thereof. The oscillator comprises a resonator element comprising highly doped silicon and an actuator for exciting said resonator body into a resonant mode having a characteristic frequency versus temperature curve. The characteristics of the resonant element and the actuator are chosen such that the curve has a high temperature trip point at a trip temperature of 85 ℃ or higher. Furthermore, the oscillator comprises a thermostatic control for maintaining the temperature of the resonator element at said high flip temperature.

Description

Furnace frequency control reference oscillator and manufacturing method thereof

Technical Field

The present invention relates to micro-electromechanical (MEMS) oscillators. In particular, the present invention relates to a temperature compensated MEMS oscillator for stabilizing its operating frequency.

Background

A conventional frequency reference oscillator includes a quartz crystal as a resonant element, which mainly determines the frequency and other characteristics of its output signal. Quartz crystals are stable, but also have some drawbacks, such as: relatively large size and difficult to integrate within electronic circuits.

Several attempts have solved the problem of using MEMS resonators as alternatives to quartz resonators (as stable frequency references). The frequency of a pure silicon crystal resonator has a strong linear temperature dependence, which makes the resonator unusable at varying temperatures. Doping of the silicon crystal can be used to somewhat homogenize the frequency-temperature curve over the intended operating range of the resonator (typically around room temperature). For example: the effect of doping on Silicon Resonators is discussed by the "piezoelectric transducer Temperature Compensated Silicon Resonators for Timing and frequency Reference Applications" (piezo electric transducer Temperature Compensated Silicon Resonators for Timing and frequency Reference Applications) written by Jaakkola, atteti at the university of alto in 2016, and WO 2012/110708a 1.

US 2012/0013410a1 discloses a method of calibrating an oscillator circuit for providing a stable level into a frequency-temperature curve for providing a stable frequency range using a polynomial temperature calibration scheme.

The above mentioned solutions can be used to provide a relatively stable resonator, but only within a relatively narrow temperature range. Furthermore, the required tuning circuitry makes the oscillator relatively complex and potentially provides an additional source of error related to the stability or phase noise characteristics of the oscillator.

A so-called OCXO, a furnace controlled crystal oscillator, is a device in which a quartz crystal is heated to a constant temperature, wherein the frequency-temperature curve has a tilt point (i.e. a so-called roll-over point). At this temperature, the first derivative of the frequency versus temperature curve is zero, and a stable oscillator output frequency can be obtained when the temperature remains near the trip point. Typically, such a flipping temperature may be, for example, about 90 degrees celsius, and thus with temperature control based on heating the crystal element in a thermally isolated enclosure (i.e., a furnace), operation below 80C temperatures may be achieved.

So-called Oven Controlled MEMS Oscillators (OCMOs) can be designed if it is possible to create MEMS resonators with a frequency-temperature curve of the switching point at elevated temperatures. MEMS resonators are much smaller in size than quartz crystals and therefore the power consumption of the oven required for OCMO is much less than that required for OCXO. Such as Vig, j, and Yoonkee Kim at IEEE proceedings of ultrasonics, ferroelectrics and frequency control, stage 60, stage 4 (4 months 2013): pages 851-53, doi:10.1109/TUFFC.2013.2634, "Low-Power of Oven-Controlled Mems Oscillators" among other benefits, including smaller overall size of The components, and faster start-up time of The Oscillators due to smaller Oven time constants.

A solution "microwave Dual mode clock (ODMC) Based on Highly Doped Single Crystal Silicon Resonators (ODMC) Based on high grounded Single Crystal Silicon Resonators" is disclosed by Yunhan Chen et al at 29 th IEEE International conference on microelectromechanical systems, 1-94, 2016, wherein a centrally anchored square plate resonator is heated to a constant temperature and the square plate resonator is driven simultaneously into two resonant modes with different resonant frequencies. The resonant frequency is tracked and the frequency difference between the two modes is used as a thermometer to provide feedback to the control loop of the resonator in order to stabilize the resonator. While allowing a relatively wide operating range, the compensation scheme is also relatively complex since both frequencies are involved (e.g. involving anchoring and control circuit implementation). However, the performance in terms of frequency stability at varying ambient temperatures has still been found to be inferior to that of the quartz-based OCXOs.

Existing OCMOs typically suffer from lower frequency stability due to their poor frequency-temperature characteristics at the switching temperature. In order for the OCMO to achieve similar stability at different ambient temperatures, more precise temperature control of the furnace is required than is currently possible with quartz-based OCXOs.

There is a need for an improved furnace controlled MEMS oscillator.

Disclosure of Invention

It is an object of the present invention to provide a furnace controlled MEMS oscillator that is more stable than existing OCMOs. In particular, it is an object to provide an oscillator with a frequency-temperature characteristic that does not impose requirements on the accuracy of temperature control of the electric furnace that are not practically feasible.

It is an object to provide an OCMO having a stable frequency over a wide temperature range, in particular up to 85 ℃.

It is an object to also provide an oscillator which is easy to implement, in particular to fulfill the requirements relating to the circuit.

The invention is based on the observation that: by increasing the average doping concentration of silicon to 9 x 10¹⁹cm^-3Or more, a resonator can be realized which has a turning point at a temperature of 85 ℃ or more and at the same time has a frequency-temperature curve at the turning point with a very low curvature. Therefore, the change in the heating temperature is only minimally reflected to the frequency of the resonator. One particular observation is that some resonators previously thought to have a frequency-temperature trip point at relatively low temperatures, in effect exhibit a frequency-temperature trip point at higher temperaturesThe other is a turning point. This other turning point can also be used as the heating temperature of the resonator to stabilize the output frequency of the oscillator.

According to a first aspect, the present invention provides a temperature compensated microelectromechanical oscillator comprising: a resonant element comprising doped silicon; an actuator for exciting the resonant element to a resonant mode having a characteristic frequency-temperature curve; and a thermostatic control for maintaining the temperature of the resonator element at the high flip temperature. The doping concentration of the resonator element is at least 9 x 10¹⁹cm^-3And is selected to provide a high temperature switching point at a switching temperature of 85 ℃ or higher in the selected resonator geometry and resonant mode.

According to an additional aspect, the high temperature trip point is the only trip point of the curve in the temperature range of-40 ℃ to +150 ℃. Especially at a doping concentration of 9 × 10¹⁹cm^-3To 1.3X 10²⁰cm^-3May be implemented.

Alternatively, the curve has two flip points, one or both of which are located at a temperature above 85 degrees celsius. Any of the turning points (typically the first turning point above 85 ℃) can be used as the heating point. This can be achieved in particular at a doping concentration of 1.1X 10²⁰cm^-3Or higher, such as 1.3 x 10²⁰cm^-3Or higher doping concentration.

According to one aspect, the present invention provides a temperature compensated microelectromechanical oscillator, comprising: comprising a doped silicon resonator element and an actuator for exciting the resonator body into a resonant mode having a characteristic frequency-temperature curve. The properties of the resonator element and the actuator are chosen such that the curve has at least two turning points. According to the invention, at least one of the turning points is a high temperature turning point at a turning temperature of 85 ℃ or higher. Furthermore, the oscillator comprises a thermostatic control for maintaining the temperature of the resonator element at said high flip temperature.

According to a second aspect, the present invention provides a method of manufacturing a microelectromechanical oscillator. The method comprises selecting a resonator geometry for the resonator element, selecting a resonator material comprising doped silicon, and selecting an actuator for providing the resonator element in the selected resonator geometry to oscillate in a resonant mode. According to the invention, it is evaluated whether the selected resonator geometry, resonator material, actuation means and resonant mode produce a frequency-temperature curve with a high temperature flip point at a high flip temperature of 85 ℃ or higher. If such behavior is found, the oscillator is fabricated using the selected resonator geometry, resonator material, and actuator, further providing the oscillator with a thermostatic control to maintain the temperature of the resonator element at a high flip temperature.

In particular, the invention is characterized by what is stated in the independent claims.

The present invention provides significant advantages. It has been found that the turning point of a silicon resonator can be "pushed" to a sufficiently high temperature as a heating temperature covering the entire actual temperature area of the electronic device and at the same time be flattened to provide a very stable frequency point. The key is the ultra-high doping concentration of the silicon material. Subsequently, preferred materials, resonator geometries and resonant mode combinations are exemplified.

In particular, it has been shown that the absolute value of the curvature of the frequency-temperature curve at the turning point amounts to 20ppb/C²Or lower, and even up to 10ppb/C²Or a lower level. This is in contrast to conventional oven resonators, which have a flat curvature, most preferably 50ppb/C²And is inferior by 10 times or more to the curvature in the quartz crystal used for OCXO. However, the present invention approximates the curvature and thus the frequency stability to that of quartz.

The presented frequency-temperature behavior can be achieved by using off-the-shelf resonator geometries, resonant modes and driving schemes, as will be exemplified in detail later herein.

The dependent claims relate to selected embodiments of the invention.

In some embodiments, there are two flipping temperatures in the frequency-temperature curve, at leastAn inversion temperature is "pushed" above 85 ℃ for heating. To achieve this, it is advantageous to exploit the third order temperature characteristic of some resonators. This approach differs from some conventional approaches, for example, in that the design goal is not only to minimize the TCF component to first order, but also to higher order, or to provide some (relatively narrow) temperature stable frequency region around room temperature. In some embodiments, one of the two turnover points of the frequency-temperature curve is at a temperature below 85 ℃, in which case the high temperature turnover point is typically a local maximum. This property is achieved at a doping concentration of 1.1X 10²⁰cm^-3Or higher, is particularly evident in resonators.

The high temperature flipping point is typically located at a flipping temperature of 200 ℃ or less, particularly 150 ℃ or less, such as 130 ℃ or less.

In some embodiments, the frequency-temperature curve has exactly two flip points in the temperature range of-40 ℃ to +150 ℃.

In some embodiments, the frequency-temperature curve has a smaller absolute value of curvature at a high temperature inversion point used as a heating point than at a low temperature inversion point.

In the case of a two-flip-point solution and a degenerate n-doped silicon slab resonator, the high temperature flip point is usually a local maximum. However, it is not excluded that the turning point is a local minimum, especially in case of some other material configuration.

In some embodiments, the resonator is a square plate resonator or a rectangular plate resonator.

In some embodiments, the resonator is a plate resonator having an aspect ratio different from 1. In one example, an aspect ratio of less than 2 is preferred, particularly in the case of an electrostatically actuated plate. In one example, particularly in the case of a piezoelectric actuation plate, the aspect ratio is preferably 2 ± 10%.

In some embodiments, the resonator is a beam resonator. In some embodiments, the beam is tilted with respect to the [100] silicon crystal direction.

In some embodiments, the resonant mode of the resonator is a square stretch/width stretch mode branch (including overtones). In alternative embodiments, the resonant mode is in-plane bending, out-of-plane bending, or length stretched/lamellized modal branching (including overtones). This means that the main mode occurring in the resonator belongs to said mentioned branch.

In general, the resonant mode used may be a stretching mode, such as a width stretching mode or a square stretching mode, a bending mode, such as an in-plane bending mode, a shear mode, or a mode having characteristics from two or more of these modes. These mode shapes have in particular been found to provide the required design freedom, in particular with respect to the aspect ratio of the plates, the angle of the plates with respect to the silicon crystal and the doping, in order to be able to realize in practice resonators with the required characteristics.

In some embodiments, the silicon of the resonator element is doped to at least 1.3 x 10²⁰cm^-3Or more average concentrations of the piezoelectrically actuated composite resonator.

In some embodiments, the thermostatic control is adapted to operate independently of the oscillation frequency of the resonant element. That is, there is no feedback loop from the frequency output of the resonator to the controller for adjusting the resonator temperature, but the controller uses direct (e.g., thermistor-based) temperature sensing. This ensures high stability of the oscillator.

Embodiments of the invention and their advantages are discussed in more detail below with reference to the accompanying drawings.

Drawings

Fig. 1A shows a block diagram of the key parts of the oscillator shown.

FIG. 1B illustrates a frequency-temperature diagram of an aluminum nitride film coupled MEMS resonator showing a switching temperature (T) above 85 deg.C_TO,h＝95℃)。

Fig. 2A and 2B illustrate an exemplary piezoelectrically actuated rectangular plate resonator in top and side views.

Fig. 3A to 3F show performance plots of 100 WE/SE mode resonators fabricated on a wafer at the development stage showing: initial frequency distribution (3A), temperature dependence of the resonance frequency (3B), quality factor Q (3C), figure of merit FOM (FOM 1/(2 pi F C0 Rm)) (3D), electromechanical resistance Rm (3E) and shunt capacitance C0 (3F).

Fig. 4A to 4C show graphs of three different frequency-temperature curves according to an embodiment of the invention.

Detailed Description

Define a limit

The term "frequency reference oscillator" herein refers to the entire device, including in particular the resonators, actuators and thermostats described herein.

The term "resonator (element)" herein refers to a silicon-based element suspended on a support structure so as to be able to resonate in a resonant mode. For example, the resonator may be a composite resonator comprising a differently doped silicon layer and/or any other material layer, e.g. required for piezoelectric actuation, and suspended together with the silicon body.

A "transducer" herein refers to a device for coupling acoustic waves to/from a resonator. The transducer may be part of the resonator element (e.g. piezo actuated) or the transducer may be arranged outside the resonator element (e.g. electrostatically actuated electrodes).

The "actuator" herein refers to the necessary drive and sense circuitry for operating the resonator and sensing the frequency of the resonator using the transducer.

TCF1, TCF2, and TCF3 refer to the first, second, and third derivatives, respectively, of the frequency-temperature curve (typically evaluated at a temperature of 25 ℃). The terms "slope" and "curvature" are used when referring to the first and second derivatives of the frequency-temperature curve at the switching temperature.

Here, "ppb" and "ppm" refer to parts per billion (10), respectively^-9) Or parts per million (10)^-6) Relative unit of (a).

The "turning point" refers to a local extreme of the frequency-temperature curve of the resonator. Turnover temperature (T)_TO) Is the temperature value corresponding to the turning point. Thus, at the switching temperature, harmonicThe slope of the frequency-temperature curve of the resonator is zero so that near the turnover temperature, the change in temperature is minimally reflected in the frequency of the resonator. In the present invention, at least one switching temperature higher than 85 ℃ is utilized in order to be able to achieve stability above the normal operating temperature without a cooling arrangement.

"degenerate doping" herein means doping to 9X 10¹⁹cm^-3Or higher impurity concentrations. For example, the dopant may be phosphorus or some other n-type agent.

Description of selected embodiments

Fig. 1A shows a resonator 11 placed inside a furnace 15. The temperature of the furnace 15 is regulated by the thermostat 13. The readout signal of the oscillator is obtained at output 19.

Next, a resonator with two switching temperatures, at least one of which is higher than 85 ℃, and capable of providing a frequency-temperature curve of significantly less than 50ppb/C at the switching point, will be discussed²Absolute curvature value of (a).

FIG. 1B shows a silicon-free substrate having a silicon doping concentration greater than 1.3X 10²⁰cm^-3An example of a frequency-temperature curve measured by the rectangular plate resonator of (1). It is noted that the frequency-temperature curves illustrated herein have two flip temperatures T according to the present invention_TO，I、T_TO，hOne at about 40 ℃ and one at about 95 ℃. The latter will be selected as the target temperature for the furnace. At a temperature T_TO，hAt a higher turning point, the estimated curvature is less than 10ppb/C in order²。

The resonator may be a composite structure, e.g. comprising a stack of different materials. In particular, the resonator may comprise a plate comprising a first layer and a second layer on top of the first layer, the layers having different TCF characteristics. In one embodiment, the linear TCFs of the first and second layer structures have opposite signs. In particular, the second layer of the composite may be a piezoelectric actuation layer.

In some embodiments, the resonator element comprises a silicon body, a piezoelectric layer on the body and an electrode layer on the piezoelectric layer, wherein the actuator is electrically connected to the electrode layer and the silicon body for exiting the selected resonance mode. The piezoelectric actuator plate resonator has been found to exhibit advantageous temperature characteristics according to the invention and, when used as part of an oscillator, exhibits low phase noise and can be tuned well to the precise center frequency required. Furthermore, the temperature characteristics of the piezoelectric layer can be advantageously used to provide the advantage of the second and third order temperature coefficients of frequency (TCF2 and TCF3) over the first order term (TCF1), thereby providing a high temperature trip point.

Fig. 2A and 2B illustrate a piezo-actuated composite resonator having a thin film transducer layer 44 superimposed on a rectangular silicon body 42 to provide strong coupling and low phase noise when the resonator is used as part of an oscillator. The membrane is typically a piezoelectric AIN layer. On top of the membrane there is an additional electrode layer 46, such as a molybdenum layer. Silicon body 44 may serve as the other electrode. In some embodiments, one or more additional layers are also provided, such as a layer of passivation material that can render the underlying material chemically inactive when processed on top of other layers.

Such piezoelectric actuation is discussed in more detail in, for example, Jaakkola, a. et al, publication (IUS 2008, 717-.

Alternatively, the resonator may be electrostatically actuated. Electrostatic actuation provides a weaker coupling between the resonator and the transducer, but the coupling has better long-term stability. The principles discussed herein are equally applicable to electrostatically actuated single crystal resonators and composite resonators.

The resonator may be, for example, in the shape of a plate (such as a rectangular plate or beam). The length direction of the plate or beam may be at an angle of 0-45 degrees with respect to the [100] crystallographic direction of the silicon material. Both of these geometric parameters (i.e., aspect ratio and angle) can be adjusted along with the material parameters and the modal branch or branches used to produce a switching temperature above 85 ℃.

The design and manufacturing process of the present oscillator may include (in any relevant order or in an iterative process): the method comprises the steps of selecting a resonator geometry, selecting a resonator material comprising doped silicon, and selecting an actuation means capable of causing the resonator to oscillate in a selected resonance mode. For example, any resonant mode may first be selected to exhibit a frequency versus temperature curve with a positive TCF 1. In one example, a plate geometry length extension mode (first order or any higher order LE mode) is selected. Then, a plate geometry and/or plate material (stack) may be selected that results in a TCF1 of zero or near zero. For example, the plate aspect ratio and/or angle with respect to a silicon crystal, and/or the thickness of the piezoelectric actuation layer on top of the silicon plate may be selected. Finally, the doping concentration of the silicon is chosen such that the second and third order performance of the resonator is superior to the first order performance.

Then, it is evaluated whether the selected resonator geometry, resonator material, actuation means and resonance mode result in a frequency-temperature curve with at least two turning points, wherein at least one turning point is a high temperature turning point at a turning temperature of 85 ℃ or higher. The evaluation may be based on simulations or experiments. In the affirmative case, an oscillator with such a resonator is manufactured, and a thermostat control is also provided to the oscillator for keeping the temperature of the resonator element at a high flipping temperature.

In some embodiments, the resonant modes are selected such that as the in-plane aspect ratio of the plate resonator (i.e. the ratio of the length of the plate resonator to its width) and/or its angle relative to the [100] crystal orientation of the silicon material varies, the properties of the resonator vary according to the variation in aspect ratio and/or angle. For example, the characteristics of interest are the resonant frequency and the temperature coefficient of the frequency (i.e., TCF1, TCF2, and higher order coefficients), and the electromechanical coupling strength of the transducer for excitation and sensing. Among the various possible aspect ratios or angles, selecting one possible aspect ratio or angle along with other design parameters results in a high switching temperature.

To give some practical examples, the resonator may be a composite width stretched/square stretched resonator, where the aspect ratio or composite in-plane bend/length stretched plate or beam resonator is selected, among other parameters, to produce a high switching temperature. These examples are discussed in more detail below.

Piezoelectric transducer temperature compensated Silicon Resonators for Timing and Frequency reference applications (piezoelectric transducer temperature compensated Silicon Resonators for Timing and Frequency reference applications) and US 2016/0099704, written by Jaakkola, atteti, 2016 at alto university generally discuss the second order temperature characteristics of Resonators below 85 ℃. When the n-type doping concentration is more than 1.1 multiplied by 10²⁰cm^-3The second order temperature coefficient TCF2 of the silicon resonator may reach positive values at room temperature. Linear TCF (TCF1) and second order temperature coefficient TCF2 can be close to zero at the same time with certain configurations involving doping levels and resonator geometry, and as doping continues to increase, TCF2 reaches a positive value, which is visible as an upwardly opening parabola in the frequency-temperature curve between-40 and +85 ℃. However, it has now been found that at high temperatures (above 85 ℃), the curve deviates from an upwardly opening parabola to and "curves downwardly". In other words, the frequency-temperature curve is not completely described by a second order polynomial, but rather has a pronounced third order behavior. This third order effect, or "bowing down," of the frequency-temperature curve results in a low curvature local maximum on the frequency-temperature curve above 85 ℃, as shown in fig. 1B, which makes the resonator suitable for heating to stabilize its frequency over the ambient temperature range of various electronic products.

An exemplary way to obtain a frequency-temperature characteristic curve with two flip points for a silicon resonator is presented below. Therefore, this approach can be applied to a semiconductor device having an average doping concentration of 1.1 × 10²⁰cm^-3Or higher (especially 1.3X 10)²⁰cm^-3Or higher) and may or may not have additional layers of materials such as piezoelectric and metal layers associated with piezoelectric actuation. This approach is based on using width stretch/square stretch (WE/SE) and in-plane bend (IFP1), out-of-plane bend (OPF1) or stretch/lamei (LE/lamee) modal branching (as described in the above articleThe characteristics of (1).

WE/SE branch: for a plate resonator with length and width, there is one square stretch/width stretch mode branch. By moving the branches from aspect ratio 1 to a higher aspect ratio, a configuration can be found in which TCF1 is close to zero. Using this aspect ratio, not only is TCF1 zeroed, but the remaining (positive) TCF2 and (negative) TCF3 result in a third order frequency-temperature curve with two switching temperatures, similar to those in fig. 1B or fig. 3B, according to the present invention. The examples of fig. 1B and 3B are measured from the device created in the manner described above. The resonator is a piezoelectrically actuated 20-MHz resonator on the SE-WE mode branch and the dimensions of the resonator are as follows: the composite resonator is formed by doping with a concentration greater than 1.3 × 10²⁰cm^-3A 20 micron thick phosphorus doped silicon layer, a1 micron thick aluminum nitride layer (AIN), and a 0.3 micron molybdenum layer as the top electrode. The resonator has a rectangular shape and has a width and length of 188 microns and 378 microns. It should be noted that by adjusting the in-plane dimensions of this design, and keeping the ratio between the thicknesses of the material layers constant, resonators with a wide frequency range can be produced.

The optimum aspect ratio for the above case of piezo-actuated resonators has been found to be close to 2 (length to width). Since the optimum aspect ratio depends on the exact doping concentration, the thickness of the resonator, and possibly other material layers added that increase their own contribution to TCF1, the actual usable aspect ratio may deviate from 2 by as much as 10%, and typically 5% by as much. Other layers of material have less effect on TCF2 and TCF 3. The optimum aspect ratio in each case can be found by experimentally testing resonator designs with different aspect ratios, varying in small steps, or correspondingly by simulations.

In particular, for a similar resonator using electrostatic actuation (from the SE/WE mode branch), as an alternative to piezoelectric actuation, there will not be any additional layers of material other than silicon, so the optimal aspect ratio will be less than 2 (i.e. between 1 and 2).

Therefore, in general, the aspect ratio of the resonator is different from 1.

As a result of the experiment, fig. 1B and fig. 3A to 3F also describe the feasibility of the present invention for industrial use.

The properties of in-plane bending (IFP1), out-of-plane bending (OPF1), or length-stretched/lamellized (LE/lamme) modal branching can be exploited in a similar manner to the properties of the WE/SE branching described above. The parameter to be changed here is the alignment of the beam resonator with respect to the [100] crystal orientation, not the aspect ratio of the resonator.

By moving in small steps over IPF1, IPF2, or LE modal branches aligned at an angle to the [100] direction, a configuration can be found where TCF1 is close to zero. In accordance with the present invention, the remaining (positive) TCF2 and (negative) TCF3 cause a third order frequency-temperature curve with two flipping temperatures, similar to those in fig. 1B or 3B, in this configuration.

It should be noted that both the in-plane aspect ratio and the angular alignment direction of the resonator can be varied simultaneously to find a configuration that gives rise to a third order frequency-temperature curve with two flip temperatures, similar to those in fig. 1B or 3B.

The exact deviation of the beam direction from the [100] crystal orientation depends on the thickness of the resonator, and possibly the addition of other layers of material that add their own contribution to TCF 1. Other layers of material have less effect on TCF2 and TCF 3.

In summary, in some embodiments, the resonator element comprises a dielectric material having a thickness of 1.3 × 10²⁰cm^-3Or a silicon-based layer of n-type doping concentration above, an aluminum nitride transducer layer and a conductive electrode layer superposed on each other. The element is in the shape of a plate or beam whose geometry produces essentially zero TCF1, positive TCF2, and negative TCF3, TCF1, TCF2, and TCF3 bringing one of the turning points of the frequency-temperature curve of the resonator to a high temperature range.

According to a particular example, the resonator has the characteristics of the resonator disclosed in the finnish patent application 20165553, which is not yet disclosed.

The exact turning temperature can be adjusted as desired by the design and manufacturing process. For example, the flipping temperature may be made higher by moving on the modal branch, such as moving towards a higher aspect ratio on the SE-WE modal branch. Similarly, the switching temperature can be made higher by bending in-plane (IFP1), bending out-of-plane (OPF1), or moving on the length stretch/lame modal branch that is closely aligned with the [100] direction. In addition, a thinner additional material layer with negative TCF1 results in a higher switching temperature. Such a layer may be, for example, a piezoelectric layer or a top electrode layer. The possibility of adjusting the switching temperature is advantageous for the industrial production of the present oscillator.

Depending on the exact design choice, one can achieve (doping concentration c ═ 9 × 10) as illustrated in fig. 4A¹⁹To 1.3X 10²⁰cm^-3) At a single high temperature flip point in the frequency-temperature curve, one can achieve (c) as illustrated in fig. 4B>1×10²⁰cm^-3) A curve with two high temperature flip points in the frequency versus temperature curve, or it may be implemented as shown in FIG. 4C (C)>1.1×10²⁰cm^-3) Has a high temperature trip point and a low temperature trip point. The concentration limits in these cases overlap because, for example, although most of the characteristics of the frequency-temperature curve are determined by the doped silicon characteristics, the added material layers can contribute themselves to the frequency-temperature curve. In each case, a 20ppb/C turnover point can be achieved at high temperatures²Or lower low curvature.

The present resonator may be the only resonator in the oscillator (single resonator oscillator) or may be one of a plurality of resonators (multi-resonator oscillator).

The thermostat controller herein preferably includes a heater (such as a resistive heater) that is placed in proximity to the resonator. In addition, there is also a temperature sensor, such as a thermistor, for measuring the temperature of each resonator whose temperature is to be regulated; and a control circuit capable of setting the temperature of the resonator to a predetermined value using the heater.

The resonator is placed in a micro-oven, which refers to a thermally isolated space containing a heater and a temperature sensor. The sensor may be a single point or multi-point sensor, in which case the temperature values may be averaged from several locations within the furnace, or the thermostat controller may use the temperature values from several locations to implement more complex control functions.

The drive circuit and/or the thermostatic control circuit of the resonator can be placed inside the oven, if required, which can also be the same as the resonator oven. This may further improve the accuracy and stability of the oscillator.

Reference list

Patent document

US 2012/0013410 A1

US 2016/0099704 A1

WO 2012/110708 A1

Non-patent document

Vig, J., Yoon kee Kim, "Low Power potential of furnace-Controlled Mems Oscillators (The Low-Power of Oven-Controlled Mems Oscillators)", IEEE ultrasonic, ferroelectric and frequency control bulletin, No.4 (4 2013), 851-53. doi:10.1109/TUFFC.2013.2634

Yunhan Chen et al, "microwave dual-Mode clocks Based on Highly Doped Single-Crystal silicon resonators (ODMC) Based on high throughput Single Crystal silicon detectors", IEEE 29 th International conference on MEMS, pp 91-94, 2016, 1 month.

Jaakkola, a et al, "piezoelectric transducer single Crystal-Silicon Plate Resonators," IEEE sonography seminar, 2008, IUS 2008, 717-20, 2008.

Jaakkola, atteti, "piezoelectric transducer Temperature Compensated Silicon Resonators for timing and Frequency Reference Applications (piezo electric driven Temperature Compensated Silicon Resonators for timing and Frequency references)", the paper by the university of alto, 2016. http:// urn.fi/URN: ISBN: 978-.

Claims

1. A temperature compensated microelectromechanical oscillator comprising:

-a resonator element comprising a doping to at least 9 x 10¹⁹cm^-3Of the average doping concentration of silicon,

-an actuator for exciting the resonator element into a resonant mode having a characteristic frequency-temperature curve with a high temperature trip point at a trip temperature of 85 ℃ or higher, an

-a thermostatic control for maintaining the temperature of the resonator element at the high flipping temperature.

2. The oscillator according to any one of the preceding claims, wherein the doping concentration is 9 x 10¹⁹cm^-3To 1.3X 10²⁰cm^-3And the high temperature trip point is the only trip point within the temperature range-40 ℃ to +150 ℃.

3. The oscillator according to any one of the preceding claims, wherein the doping concentration is at least 1.1 x 10²⁰cm^-3And the frequency-temperature curve has two turnover points, one of the two turnover points being the high temperature turnover point.

4. The oscillator of claim 3, wherein one of the two trip points is a low temperature trip point at a temperature below 85 ℃.

5. The oscillator of claim 4, wherein an absolute value of a curvature of the frequency-temperature curve at the high temperature rollover point is less than an absolute value of a curvature at the low temperature rollover point.

6. The oscillator according to any one of claims 3 to 5, wherein the frequency-temperature curve has exactly two flip points in the temperature range of-40 ℃ to +150 ℃.

7. The oscillator according to any one of the preceding claims, wherein the high temperature trip point is a local maximum.

8. The oscillator of any one of claims 1 to 6, wherein the high temperature trip point is a local minimum.

9. The oscillator according to any one of the preceding claims, wherein the absolute value of the curvature of the frequency-temperature curve at the high temperature flipping point is 20ppb/C²Or less, in particular 10ppb/C²Or smaller.

10. The oscillator according to any one of the preceding claims, wherein the high temperature trip point is at a trip temperature of 200 ℃ or lower, in particular at 150 ℃ or lower, such as 130 ℃ or lower.

11. The oscillator of any one of the preceding claims, wherein the resonator element comprises:

-a silicon body having a 1.3 x 10²⁰cm^-3Or an n-type doping concentration of the above,

a piezoelectric transducer layer, such as an aluminum nitride layer, on the body,

-an electrode layer on the piezoelectric layer, and

wherein the actuator is electrically connected to the electrode layer and the silicon body for exciting the resonant mode.

12. The oscillator of any one of the preceding claims, wherein

-the resonator element comprises a degenerately doped monocrystalline silicon body,

-the oscillator comprises an electrostatic transducing electrode functionally coupled to the body,

wherein the actuator is electrically connected to the electrodes for exciting the resonant mode.

13. The oscillator according to any one of the preceding claims, wherein the resonator elements are plate elements, such as rectangular plate elements, having an in-plane aspect ratio different from 1.

14. The oscillator of any one of the preceding claims, wherein the resonant mode is in a square stretch/width stretch mode branch.

15. The oscillator according to any one of the preceding claims, wherein the resonant modes are in-plane bending, out-of-plane bending, or length-stretched/lamelliform branches.

16. The oscillator according to any one of the preceding claims, wherein the resonator element is shaped as a plate or a beam, the geometry of the resonator element being such that the element in the resonant mode has a TCF1 of substantially zero and TCF2 and TCF3 characteristics providing at least two turning points, one of which is the high temperature turning point.

17. The oscillator according to any one of the preceding claims, wherein the doped silicon has a [100] crystal orientation and the resonator element has at least one principal axis having an angle with respect to the [100] crystal orientation, wherein the angle is 0 to 45 degrees, such as 1 to 45 degrees.

18. The oscillator according to any one of the preceding claims, wherein the thermostatic control is adapted to operate independently of the oscillation frequency of the resonant element.

19. A method of fabricating a microelectromechanical oscillator comprising

-selecting a resonator geometry for the resonator,

-selecting a resonator material comprising a material having an average doping concentration of 9 x 10¹⁹cm^-3Or a higher doping of the silicon or of a higher doping,

-selecting an actuation means for providing the resonant material in the resonator geometry to oscillate in a resonant mode,

-evaluating whether the selected resonator geometry, resonator material, actuation means and resonance mode result in a frequency-temperature curve with a high temperature flip point at a high flip temperature of 85 ℃ or higher, and, in the affirmative case,

-manufacturing the oscillator using the resonator geometry, the resonator material and the actuation means, wherein the manufacturing further comprises providing a thermostatic control for maintaining the temperature of the resonator element at the high flipping temperature.

20. The method of claim 19, wherein the selection of the resonator geometry comprises selecting a rectangular plate geometry or a beam geometry.

21. The method of claim 19 or 20, wherein the selecting of the resonator material comprises selecting a stack of materials comprising: comprises an average doping concentration of 1.3 × 10²⁰cm^-3Or higher doped silicon, and a second layer comprising a piezoelectric material.

22. The method according to any one of claims 19 to 21, wherein the resonant modes are in square stretch/width stretch modal branches or in-plane bend, out-of-plane bend or length stretch/lame modal branches.